Photoplethysmographic (PPG) measuring of heart rhythm uses a light source such as a Light Emitting Diode (LED) which irradiates light into a subject's skin. Light reflected from deeper layers in the skin is detected by a photodetector. Alternatively, light transmitted through the skin and tissue may be measured. As the blood flow through the skin varies with every heartbeat, so does the reflection or transmission of the light in or through the skin vary. The variation is detected by a control circuit and a heart rhythm may be determined from this variation.
PPG sensors can be integrated in wearables such as cuffs arranged for heart rhythm measurement, wristbands of smartwatches and the like.
The amplitude of a PPG signal from such PPG sensor in reflection and transmission mode depends on the pressure exerted by the sensor and the LED to the skin. Normally there is an optimal pressure wherein the amplitude of PPG signals is maximum. There is however for the wearer no easy way to know if the PPG signal has the highest amplitude when the sensor is attached on the skin. A wearer does not have a feedback on how tight they should wear the PPG sensor. Usually wearer instructions are vague and wearers tend to wear the sensor more tightly than is required. This may lead among other things to skin irritation. A wearer can use trial and error to find out which tightness works best. Alternatively a varying tightening force can be applied using an actuator until the desired signal strength is obtained.
Such strategies however can be time consuming and bulky, whereas a wearer may want a measurement on short notice. Moreover, comfortability decreases with contact pressure. When the fastening band is loose (e.g. as a wearer would normally wear a watch) there is a bad contact pressure and no PPG signal can be measured. If the fastening band or bracelet is too tight this will lead to pain because the skin is compressed. In addition, a too tight fastening band will have a detrimental effect on the PPG signal since the compressed capillaries will decrease the blood circulation. The optimum contact pressure is somewhere in the middle, not more than what is comfortable and enough for a good PPG signal.